Field of Endeavor
The present invention relates to the technology of turbo machines, such as compressors or gas turbines, or others. It relates more specifically to a self-adjusting device for controlling the clearance, especially in a radial direction, between rotating and stationary components of a thermally loaded turbo machine.
Brief Description of the Related Art
Minimization of clearances, particularly radial ones between stationary and rotating parts of a turbo machine, e.g., steam- or gas turbines, jet-engines, compressors, and turbochargers, during operation is essential for minimizing flow-losses and subsequently maximizing the efficiency of such engines.
For illustrative purposes, FIG. 1 shows an example of a turbine configuration, the turbo machine stage 10 including a blade 14 and a vane 15, mounted to a rotating shaft 12 (rotating with rotational velocity Ω around an axis 11) and a housing part 13, respectively, whereby a fluid channel (hot gas channel for gas turbines) 16 is defined between the shaft 12 and the housing 13. By minimizing the radial clearances Cb (blade clearance) and Cv (vane clearance) leakage flows across the blade- and vane-tips and, consequently, flow-losses can be reduced. Radial clearances may be formed by cylindrical or conical surfaces of the respective rotor and stator components. However, although having basically a cylindrical or conical shape, every part might have additional shape features.
Due to the relative movement, e.g., between the blade tip of blade 14 and the housing 13, it is not possible to set the radial clearance Cb to zero. Contact between these parts during operation may lead to damage or even destruction of the entire engine.
In general, by designing such an engine, the radial clearance during operation (“hot-clearance”) is determined by a number of factors, which need to be considered with respect to the assembly clearances (“cold clearances” under standstill conditions for a “cold” engine, usually meaning ambient temperature):                Manufacturing tolerances        Assembly tolerances        Deformation of shaft 12 and housings 13 under steady-state operation conditions (e.g., ovalization of the casing from the demand circular shape)        Blade/Vane expansion during operation due to thermal growth and centrifugal forces        Time-dependent deformations and relative movements during engine transient operation, like start-up and shut-down of the engine.        
The latter two factors, transient and steady state thermal expansions, are driven by the temperature differences within the engine. The typical temperature distribution within, e.g., a turbine, leads to a non-constant temperature profile at a certain axial position with respect to the radial direction (temperature gradient in the radial direction). The highest temperature is assumed to be in the fluid channel 16, where the blades 14 and vanes 15 are placed. The ambient temperature is TA. FIG. 3 shows the spatial (radial, i.e., dependent on the radius R) temperature distributions within a turbine at different points in time (t1-t6).
Radial sections in FIG. 3 are defined by constructional features of the turbine, namely the shaft 12, the blade 14, the housing 13, the foot 17 of the blade 14, a heat shield 18 opposite the tip of blade 14, a chamber 19 behind heat shield 18, and the outer surface 20 of the machine. The upper part of FIG. 3 shows possible variation of the temperature distribution T(R) along the radial direction R during cooling (T↓) at instants t3-t6 in the time domain t. The lower part of FIG. 3 shows the possible variation of the temperature distribution T(R) during heating (T↑) at instants t1-t3. In both cases a transformation temperature range is illustrated by a shadowed bar in FIG. 3. It can be seen that the variation in time of the temperature during transient operation (heating/cooling) is a maximum at the blades 14 in the fluid channel 16.
In particular, the time-dependent deformations and relative movements of the main components during engine transient operations are of significance for determining the cold standstill clearances Css and for the resulting steady-state hot or nominal clearances Cnom. The design target is to determine the cold clearances so that, during steady-state conditions, the resulting hot clearances Cnom are minimized. However, due to the different geometrical dimensions, materials and heat capacities of blading, housings, and shafts during warm-up (start-up) or cool-down (shut-down) of the engine, the minimum hot clearance may not appear at the hot steady-state condition, where the minimum clearances are desirable. Furthermore, taking into account that the engine may also experience rapid load-changes or may be started, when major components are still hot from a previous operation period, the minimum possible clearances during operation (“pinch-point”, Cpp in FIG. 2) may appear sometimes during the transient engine operation (in a start-up condition S-U but not in a steady-state condition S-S, see FIG. 2(a)). In that case, it is necessary to further increase the cold-clearances to avoid “hard” contact between stationary and rotating parts during such transient operation, which then results in a larger than desired hot clearance under steady-state conditions.
For example, during start-up of the engine, the thermal expansion of the blading is typically much faster than those of casing parts or shaft, which has higher thermal inertia due to the bigger mass in comparison to the blading. Thus, heating up and thermal expansion of shaft or structural parts (dash-dotted line in FIG. 2(b)) continue even after the working fluid temperature (dotted line in FIG. 2(b)) reached the nominal level Tn, to finally reach a nominal (metal) level Tmn.
Furthermore, the casings and shafts are typically not directly exposed to the hot gases. This fact leads to so called pinch-point, which means the time instant, when the radial clearance reaches its minimal value (Cpp). Therefore, for the nominal steady-state operation condition, the resulting nominal clearance Cnom must involve a clearance value covering pinch-point and further thermal expansions, which are analytically determined based on thermal boundary conditions, dimensions and material properties of the rotating and stationary components.
Known measures to minimize the flow-losses caused by remaining hot-clearances are e.g., introducing shrouds on the tips of the blade and vane airfoils. In order to minimize the flow through the annular gap formed by the shroud and the casing or rotor, rows of fins are applied onto the rotating part, while the surface of the stationary part may be flat or stepped, overall forming a labyrinth seal.
Furthermore, honeycombs may be attached on the surface of the stationary part, allowing the fins to cut into the honeycombs during transient operation, thus forming a stepped labyrinth and further reducing leakage flows during steady-state operation. A further known measure to minimize the hot clearances is to use so called leaf- or brush-seals attached to the stationary part, which can accommodate some variation in the clearance during transient operation.
Finally, the use of a combination of abrasive features and abradable coatings on the opposite parts can be used to mitigate the effect of circumferential clearance variations, e.g., caused by ovalization of structural parts or some eccentricity of the shaft within the casing.
While all the above mentioned solutions are purely passive systems, which allow minimizing hot clearances without any adjustment to the temperature, pressure and other loadings during operation, there are also a number of other active clearance reduction measures known.
A system has been described, where the complete rotor of a gas turbine is shifted axially after the engine has reached its steady-state condition. In combination with a conical fluid flow path, this allows actively minimizing the hot turbine clearances and can be combined with any of the above mentioned passive measures. However, since the whole rotor train needs to be moved, this also results in some increase of clearances on the compressor side. Therefore, this measure will only be beneficial as long as the gains in the turbine outweigh the additional losses on the compressor side.
Instead of a general axial shaft movement, solutions for every turbine stage have been proposed by controlling either the radial thermal blade/vane expansion or a spring system, which allows for an additional radial movement of the heat shield (see for example U.S. Pat. No. 7,596,954 B2).
U.S. Patent Application Pub. No. 2009/0226327 A1 describes a sleeve made of shape memory alloys, which is assembled in the rotor disc. In terms of the surrounding temperature, the sleeve throat controls the coolant flow mass into the turbine blade. By getting less coolant, the blade expands thermally to reduce a gap between its airfoil tip and a casing. By providing more coolant, the blade contract to increase a radial gap of the turbine stage.
Document GB 2 354 290 A describes the shape memory alloy valve mounted within cooling passage of the gas turbine blade. This shape memory alloy valve allows regulating the use of coolant in response to the temperature of the component. However, GB 2 354 290 A does not claim solution for clearance control for the rotating blade and stationary vanes.
U.S. Pat. No. 7,686,569 B2 presents a system for the axial movement of the blade ring driven by a pressure differential supplied across the blade ring, the thermal expansion and/or contraction of a linkage or by a piston. A shape memory alloy can also trigger the required movement.
In general, different passive, semi-active, and active systems can be considered for clearance management between the blade 14 and housing 13 as well the vane 15 and rotor or shaft 12. Clearance Cb or Cv (FIG. 1) denotes a relative distance between the rotating and stationary component, that varies with different rates during transient operation conditions, like the start-up or shut-down of the engine, due to the different thermal inertia of the various engine components, depending on their volumes, exposure to hot or cold fluids, and the thermal properties of the alloys involved.
Because of these differences, a “hot” clearance Cnom under the steady-state operation condition needs to include the transient contribution Cnom-Cpp corresponding to the difference of the thermal expansions of the rotating and stationary components during transient operation of the engine. This difference has to be taken into account during the definition of cold-built clearance, worsening the aerodynamical efficiency of the turbo machine. The ultimate target is to keep always the radial clearance as small as Cpp during the whole time of the engine operation.